P53-DEPENDENT AND INDEPENDENT EXPRESSION OF P21 DURING CELL-GROWTH, DIFFERENTIATION, AND DNA-DAMAGE

Expression of p21 has been shown to be up-regulated by the p53 tumor s uppressor gene in vitro in response to DNA-damaging agents. However, p 21 expression can be regulated independently of p53, and here we show that expression of p21 in various tissues during development and in th e adult mouse occurs in the absence of p53 function. However, most tis sues tested did require p53 for p21 induction following exposure of th e whole animal to gamma irradiation. These results show that normal ti ssue expression of p21 to high levels is not dependent on p53 and conf irm that induction of p21 by DNA-damaging agents does require p53. p21 is expressed upon differentiation of p53-deficient murine erythroleuk emia (MEL) cells, and the kinetics of induction of p21 in this system suggest that it may be involved in the growth arrest that precedes ter minal differentiation. The gene is up-regulated in mouse fibroblasts i n response to serum restimulation but the kinetics and levels of induc tion differ between wild-type and mutant cells. Expression of p21 mess age following serum restimulation is superinducible by cycloheximide i n wild-type but not in p53 deficient cells. The increases in p21 mRNA are reflected in changes in p21 protein levels. p21 expression also ap pears to be regulated at the post-transcriptional level because modera te increases in mRNA expression, during differentiation of MEL tells a nd upon serum restimulation of fibroblasts, are followed by large incr eases in protein levels, Regulation of the mouse p21 promoter by p53 d epends on two critical p53-binding sites located 1.95 and 2.85 kb upst ream from the transcriptional initiation site. The sequences mediating serum responsiveness of the promoter map to a region containing the p roximal p53 site. p53 appears to play a critical role in p21 induction following DNA damage. Moreover, p21 can be regulated independently of p53 in several situations including during normal tissue development, following serum stimulation, and during cellular differentiation.